The present invention relates generally to digital cellular communications, and more particularly, to a maximum likelihood sequence estimation based equalization method for use in mobile digital cellular receivers.
Communication channels in the cellular environment commonly impose a combination of distorting effects on transmitted signals. Rayleigh fading, where a signal's perceived power level rises and falls rapidly over a wide range, results from the combination of signals that have traversed paths differing in length by at least a significant fraction of a wavelength (i.e., about 30 cm. for cellular). Differences in path transmission times that approach the time taken to transmit a symbol result in a second problem called delay spread.
Delay spread results in reception of multiple delayed replicas of a transmitted signal. Each Rayleigh faded replica has randomly distributed amplitude and phase, and the rate at which this complex quantity varies is constrained by the Doppler bandwidth associated with a vehicle's speed. In a frequency nonselective environment, the sampled outputs of a receiver's matched filter provides uncorrelated estimates of the transmitted data. As such, in terms of discrete time samples, the channel has exhibited an impulse response proportional to a delta function. With delay spread, on the other hand, the discrete time channel impulse response is extended to introduce energy at a number of symbol times. The effect of the channel on the transmitted signal, in turn, may be viewed as the convolution of the transmitted information with the channel's impulse response. The channel, therefore, emulates a convolutional coding process.
This leads to the possibility of estimating the transmitted information through the use of methods analogous to typical decoding of convolutional codes, i.e., maximum likelihood sequence estimation techniques. Unlike the more widely applied forward error correction decoding environment, the details of the encoding process in a reverse error correction decoding environment, are not known a priori by the receiver. Issues related to the need to estimate the form of the encoding process are addressed by this invention.
For the North American digital cellular system, a number of documents define the standards of implemented components. With respect to this invention, the following are of interest: "Dual-Mode Mobile Station--Base Station Compatibility Standard" denoted here as IS-54, [EIA/TIA Project Number 2398, Rev. A, January 1991] and "Recommended Minimum Performance Standards for 800 MHz Dual-Mode Mobile Stations", denoted here as IS-55, [EIA/TIA Project Number 2216, April 1991].
It would therefore be desirable to provide an enhancement to the processing performed by equalizers for use in mobile telephones that provides for system complexity reduction and that provides for better performance in a fading channel.